Thermo gravimetric analysis (TGA) and differential TGA (DTGA) were performed to investigate phase separation behavior of organic- inorganic hybrids. The hybrids were composed of six-arm star-shaped aliphatic poly(E-caprolactone) and methyl silsesquioxane (MSSQ). By analyzing the DTGA thermograms of the hybrids and the porogen, we could qualitatively estimate the degree of interaction between organic porogen and inorganic matrix and domain sizes of the porogen in hybrids. When porogen loading was low in the hybrids, there was only one main peak on DTGA curve and it positioned at higher temperature than that of pure porogen. This shift in decomposition temperature resulted from nano-sized porogen domains in MSSQ. As porogen loading increased over critical points, the additional peaks appeared at the lower temperatures indicated that porogen molecules began to form larger poragen domains which were confirmed by the cross-sectional FE-SEM images. The porogen with higher MW was decomposed at higher temperature but its hybrids resulted in lower decomposition temperatures due to larger domains of the porogen.